With a TGA 3d30 adapter, the server just do nothing, and the gray
X11 screnn
doesn't appear. The tga driver prints suspicious values to the xorg
log
(all 0xff) which I assumes it gets from the adapter's registers, so
it's
possible reading adapter registers don't always works the way it
should.

playing more with the MGA driver and enabling debug, I can confirm

that xf86ReadMmio8 always return 0xff. So it looks like the memory
mapped

by xf86 isn't in the card's registers or memory. indeed it's using a
strange value:
(II) memory base = 0xa7a486b7525203d
Any idea still welcome ...

I don't think Xorg's alpha-specific PCI support code works right on
NetBSD - I think you should do as sparc64 and the powerpc ports do -
use netbsdPci.c / ppc_video.c and mmap PCI resources through /dev/
ttyE0 at offset == bus address. Much less hassle. If you need PIO
access define PCI_MAGIC_IO_RANGE to something never used by PCI memory
resources, the necessary voodoo is already in place in vga_pci.c.
This of course assumes that alpha, like sparc and powerpc, accesses
PCI IO space through some special memory range. If that's not the case
please ignore the bit about PCI_MAGIC_IO_RANGE.